1. Field of the Invention
The present invention relates to a coherent mixer.
2. Description of the Related Art
A monolithically integrated 90° hybrid is discussed in “First monolithic indium phosphide based (InP-based) 90°-hybrid opto-electronic integrated circuit (OEIC) comprising balanced detectors for 100GE coherent front ends” IPRM2009, TuB2.2 (Non-Patent Literature 1) by R. Kunkel et al. The 90° hybrid is formed of an InP-based semiconductor material. When the 90° hybrid includes a 2×2 multimode interference (MMI) coupler, in order to precisely control the phase of an optical signal, a phase shifter is included in the 90° hybrid.